Grafted photoinitiators

ABSTRACT

The invention provides a method for producing a polymeric photoinitiator, of the general formula I: Polymer-[CR2-CHR-Spacer(PI) n ] m  (I) wherein m is an integer from 1-5000; n is a real number above 0 and below 5; R is as defined herein, and PI is a photoinitiator moiety. The method comprises grafting one or more photoinitiator moieties (PI), each of which comprises at least one activated alkene functional group, onto the polymer. Polymeric photoinitiators are also provided.

FIELD OF THE INVENTION

The present invention relates to polymeric photoinitiators and a method for their production. The photoinitiators may be obtained by grafting a photoinitiator moiety containing an activated alkene functional group (preferably an acrylate functional group) onto the polymer, typically using a radical initiator such as, for example, a peroxide. The invention also relates to uses of said photoinitiators and methods comprising said photoinitiators.

BACKGROUND OF THE INVENTION

Curing of coatings through ultraviolet (UV) radiation, thereby resulting in a coating for use as a gel (e.g. a hydrogel), requires efficient methods of initiating the chemical reaction responsible for the curing process. Cross-linking of polymeric material through generation of radical species upon irradiation with UV light is widely used to produce hydrogels for medical device coatings. Coating compositions with polyvinylpyrrolidone and a photoinitiator as the main constituents, which are cured with UV irradiation, are often used for producing hydrogels. The photoinitiators used in these processes can be either oligomeric or polymeric. Oligomeric photoinitiators are partially free to diffuse to the surface of the cured material, thereby rendering these substances exposed to the environment.

EP 2 060 589 discloses Si—H functional arylketones, in which the arylketone comprises a photoinitiator moiety.

U.S. Pat. No. 4,477,326 relates to polymeric photoinitiators comprising organosilane polymers.

OBJECT OF THE INVENTION

The object of the present invention is to provide polymeric photoinitiators, methods for their use, as well as means and methods for the production of these polymeric photoinitiators. Such polymeric photoinitiators may constitute the entirety of a matrix composition, or may be mixed with other components in a matrix composition.

SUMMARY OF THE INVENTION

In a broad aspect, the present invention relates to a method for producing a polymeric photoinitiator, of the general formula I

Polymer-[CR₂—CHR-Spacer(PI)_(n)]_(m)  (I)

wherein m is an integer from 1-5000; n is a real number above 0 and below 5; and PI is a photoinitiator moiety; and wherein each R group is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups), said method comprising grafting one or more photoinitiator moieties (PI), each of which comprises at least one activated alkene functional group, onto the polymer.

The method of the invention provides access to a number of polymeric photoinitiator, such as those of the general formulas II, IIa, IIb, III, IV and V:

Polymer-[CR₂—CHR—C(═O)-Spacer(PI)_(n)]_(m)  (II)

Polymer-[CR₂—CHR—C(═O)—O-Spacer(PI)_(n)]_(m)  (IIa)

Polymer-[CR₂—CHR—C(═O)—NH-Spacer(PI)_(n)]_(m)  (IIb)

Polymer-[CR₂—CHR—O-Spacer(PI)_(n)]_(m)  (III)

Polymer-[CR₂—CHR—NR¹-Spacer(PI)_(n)]_(m)  (IV)

Polymer-[CR₂—CHR-Ph-Spacer(PI)_(n)]_(m)  (V)

In the above formulae II, IIa, IIb, III, IV and V: m is an integer from 1-5000; n is a real number above 0 and below 5; Ph is an optionally substituted phenyl group, each R group is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups); R¹ is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN and halogens, and PI is a photoinitiator moiety.

The invention also relates to the use of a polymeric photoinitiator according to the invention for curing a matrix composition.

A method of curing a matrix composition is also provided. In general terms, the method comprises the steps of:

-   -   a. providing a matrix composition comprising the polymeric         photoinitiator according to the invention;     -   b. curing the matrix composition obtained in step a. by exposing         it to UV radiation.

The invention also provides a matrix composition obtainable via the above method.

Further details of the invention are provided in the following description and the dependent claims.

LEGENDS TO THE FIGURE

FIG. 1 illustrates a general motif of polymeric photoinitiators, with photoinitiator moieties pendant on a polymeric backbone.

FIGS. 2 and 3 illustrate the change in rheology of samples of 1 and 3 in Example 2 as a function of exposure time.

DETAILED DISCLOSURE OF THE INVENTION Definitions

“Optionally-substituted” means optionally-substituted with one or more substituents selected from the group consisting of C1-C25 linear, branched or cyclic alkyl, aryl, —OH, —CN, halogens, amines, amides, alcohols, ethers, thioethers, sulfones and derivatives thereof, sulfonic acid and derivatives thereof, sulfoxides and derivatives thereof, carbonates, isocyanates, nitrates, acrylates. Preferably the one or more substituents are selected from the group consisting of —OH, —CN, halogens, amines, amides, alcohols, ethers, thioethers, sulfones and derivatives thereof, sulfonic acid and derivatives thereof, sulfoxides and derivatives thereof, carbonates, isocyanates, nitrates, acrylates. Most preferably, the substituent is selected from the group consisting of —OH, —CN, halogens, amines, amides, alcohols, ethers, thioethers, sulfones and derivatives thereof, sulfonic acid and derivatives thereof, and sulfoxides and derivatives thereof.

Hydrophilic

A material is described as hydrophilic if it has a natural affinity to water. Hydrophilic materials are defined as those which have a contact angle with water of less than 90°, preferably less than 80°, more preferably less than 75° and most preferably less than 50° (see ASTM D7334-08) measured with an advancing contact angle measurement. In short, the method for measuring the advancing contact angle of a water drop on a surface, is done by deposition of the water droplet (˜5-20 μL) controlled in size within 0.1 μL using a hypodermic syringe. A goniometer is then adjusted such that the interior angle of each of the two points of contact of the drop can be determined. Two angle measurements (one on each drop edge) of three drops on the specimen is determined and the contact angle for the specimen is the average of these six angle measurements.

A hydrophilic polymer is likely to contain atoms with high electronegative values such as oxygen and nitrogen. Materials which are hydrophilic according to the above definition will also have an affinity for short-chain (e.g. C1-C8) alcohols and glycerol. Specific examples of hydrophilic polymers are polyethylene oxides, polyvinylacetates, polyvinylpyrolidones, amine functional polymers e.g. poly(2-ethyl-2-oxazoline), acrylics, polyethers, polyalkylethersulfonate and polyvinyl alcohols.

Hydrophilic Gels

A gel is an interconnected, rigid network with pores of submicrometer dimensions and polymeric chains whose average length is greater than a micrometer. The term “gel” is discussed in detail in Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, N.Y., 1953; Chapter IX.

A definition of a gel is provided in Polymer Gels and Networks, 1 (1993), 5-17: A gel is a soft, solid or solid-like material of two or more components one of which is a liquid, present in substantial quantity. Solid-like gels are characterized by the absence of an equilibrium modulus, by a storage modulus, G′(ω), which exhibits a pronounced plateau extending to times at least of the order of seconds, and by a loss modulus, G″(ω), which is considerably smaller than the storage modulus in the plateau region.

In the interest of characterizing the efficiency of a photoinitiator in cross-linking polymeric matrices, the transition from a liquid to a solid material is of importance. Liquids are characterized by having G″(ω)>G′(ω) and correspondingly, solids are characterized by G″(ω)<G′(ω). The transition from liquid to solid, often referred to as the gel-point, is defined as when G″(ω)=G′(ω). The cure time defined as the time from initiation of a curing process to when G″(ω)=G′(ω) or tan δ=1 is a characteristic measure of the efficiency of a photoinitiator in a specific matrix composition.

SPECIFIC EMBODIMENTS OF THE INVENTION

The present invention provides polymeric photoinitiators, and methods for their fabrication and use.

In a first aspect, the invention provides a method for producing a polymeric photoinitiator, of the general formula I:

Polymer-[CR₂—CHR-Spacer(PI)_(n)]_(m)  (I)

-   -   wherein m is an integer from 1-5000;     -   n is a real number above 0 and below 5; and     -   PI is a photoinitiator moiety;         and wherein each R group is a substituent independently selected         from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl,         C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines         (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25         alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′         and R″ are alkyl groups, suitably C1-C25 alkyl groups).

The method comprises grafting one or more photoinitiator moieties (PI), each of which comprises at least one activated alkene functional group, onto the polymer. The resulting polymeric photoinitiators are thermoplastic. As such, they become less viscous under higher shear rate, making them easier to process in an extrusion process.

As the photoinitiators are bound within the matrix composition after curing, the likelihood of photoinitiators of low molecular weight leaching from the surface of the cured material is reduced.

Each R group is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups). Preferably, R is independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, preferably hydrogen.

Suitably, the polymer is thermoplastic. Suitably, the polymer is selected from the group consisting of polyolefines, polyacrylates, polyesters, polyurethanes, polyamides, polyalkyloxides, polydialkylsiloxanes or various copolymers comprising blocks or repeatable units of these polymers. Preferably, the polymer is selected from the group consisting of polyolefines, polyacrylates, polyesters, polyurethanes, polyamides, polyalkyloxides and polydialkylsiloxanes. Most preferably, the polymer is a polyolefin or polyalkyloxide, preferably a polyalkyloxide. The polymer suitably has a molecular weight in the range of 50-500,000 Da.

The spacer is suitably selected from the group consisting of a single bond, C1-C25 linear alkylene, C3-C25 branched alkylene, C3-C25 cycloalkylene, arylene, heteroarylene, amines, amides, alcohols, ethers, esters, thioethers, sulfones and derivatives thereof, sulfonic esters and derivatives thereof, sulfoxides and derivatives thereof and carbonates, ketones and esters. Preferably, the spacer is selected from the group consisting of C1-C25 linear alkylene, C3-C25 branched alkylene, C3-C25 cycloalkylene, and esters.

The spacer may be a polymeric entity. The spacer may be selected from the group consisting of polyolefines, polyacrylates, polyurethanes, polyesters, polyamides, polydialkylsiloxanes, polyalkyloxides or various copolymers comprising blocks or repeatable units of the mentioned polymers. If polymeric, the spacer may have a molecular weight in the range of 50-500,000 Da.

In Formula (I), PI is a photoinitiator moiety.

In the polymeric photoinitiator of Formula (I), m is an integer from 1-5000, preferably from 1-3000, more preferably from 1-1000. n is a real number above 0 and below 5.

The index n refers to a statistical average, and m is the total number of photoinitiator containing moieties attached to the polymeric backbone. Thus a total of (n×m) photoinitiator moieties are present in the polymeric photoinitiator. An example of the nomenclature is given in Scheme 1.

In the present invention, M_(w) (the weight averaged molecular weight) is used to characterize the polymeric photoinitiators. Efficiency of the polymeric photoinitiator is—among other things—related to how well the photoinitiator is blended with the gel-forming polymer(s) or monomer(s). Amongst important parameters in this respect is the molecular weight of the photoinitiator. A molecular weight which is too high does not allow for good miscibility of the polymeric photoinitiator with other components of the matrix composition. Important for the present invention is the miscibility of the polymeric photoinitiator with the other components in the matrix composition, when considering a two-component system. In particular, if the chemical nature and molecular weight of the polymeric photoinitiator and the gel-forming polymer(s) are markedly different, a poor miscibility is obtained, which in turn results in a matrix composition that is difficult to cure.

Sub-Structures

The invention provides polymeric photoinitiators with sub-structures falling within the scope of Formula (I) above.

The polymeric photoinitiator may have the general formula II:

Polymer-[CR₂—CHR—C(═O)-Spacer(PI)_(n)]_(m)  (II)

-   -   wherein m is an integer from 1-5000;     -   n is a real number above 0 and below 5;     -   each R group is a substituent independently selected from         hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25         cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g.         —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl         groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″         are alkyl groups, suitably C1-C25 alkyl groups);     -   and PI is a photoinitiator moiety.

Another sub-structure for the polymeric photoinitiator has the general formula IIa:

Polymer-[CR₂—CHR—C(═O)—O-Spacer(PI)_(n)]_(m)  (IIa)

-   -   wherein m is an integer from 1-5000;     -   n is a real number above 0 and below 5;     -   each R group is a substituent independently selected from         hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25         cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g.         —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl         groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″         are alkyl groups, suitably C1-C25 alkyl groups);     -   and PI is a photoinitiator moiety.

A further sub-structure for the polymeric photoinitiator has the general formula IIb:

Polymer-[CR₂—CHR—C(═O)—NH-Spacer(PI)_(n)]_(m)  (IIb)

-   -   wherein m is an integer from 1-5000;     -   n is a real number above 0 and below 5;     -   each R group is a substituent independently selected from         hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25         cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g.         —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl         groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″         are alkyl groups, suitably C1-C25 alkyl groups);     -   and PI is a photoinitiator moiety.

The polymeric photoinitiator may have the general formula III:

Polymer-[CR₂—CHR—O-Spacer(PI)_(n)]_(m)  (III)

-   -   wherein m is an integer from 1-5000;     -   n is a real number above 0 and below 5;     -   each R group is a substituent independently selected from         hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25         cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g.         —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl         groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″         are alkyl groups, suitably C1-C25 alkyl groups);     -   and PI is a photoinitiator moiety.

The polymeric photoinitiator may also have the general formula IV:

Polymer-[CR₂—CHR—NR¹-Spacer(PI)_(n)]_(m)  (IV)

-   -   wherein m is an integer from 1-5000;     -   n is a real number above 0 and below 5;     -   each R group is a substituent independently selected from         hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25         cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g.         —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl         groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″         are alkyl groups, suitably C1-C25 alkyl groups);     -   R¹ is a substituent independently selected from hydrogen, C1-C25         linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl,         heteroaryl, —OH, —CN and halogens;     -   and PI is a photoinitiator moiety.

The polymeric photoinitiator may have the general formula V

Polymer-[CR₂—CHR-Ph-Spacer(PI)_(n)]_(m)  (V)

-   -   wherein m is an integer from 1-5000;     -   n is a real number above 0 and below 5;     -   Ph is an optionally substituted phenyl group     -   each R group is a substituent independently selected from         hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25         cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g.         —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl         groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″         are alkyl groups, suitably C1-C25 alkyl groups);     -   and PI is a photoinitiator moiety.

According to sub-structure (V), the Ph group is substituted with one or more substituents independently selected from C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups). Suitably, the phenyl group is unsubstituted.

Of the above sub-structures of the general formulas II, IIa, IIb, III, IV and V, those of formula II (including IIa and IIb) are preferred. This is because the alkene conjugated with a carbonyl group is more reactive in the grafting process. Sub-structures of the general formula IIa are most preferred, as they have the additional benefit of compatibility with acrylate-based matrix compositions, and are relatively simple to synthesise.

Photoinitiator and Photoinitiator Moieties

As set out above, PI is a photoinitiator moiety. In the present invention, a photoinitiator is defined as a moiety which, on absorption of light, generates reactive species (ions or radicals) and initiates one or several chemical reactions or transformation. One preferred property of the photoinitiator is good overlap between the UV light source spectrum and the photoinitiator absorption spectrum. Another desired property is a minor or no overlap between the photoinitiator absorption spectrum and the intrinsic combined absorption spectrum of the other components in the matrix composition.

Suitably, the photoinitiator moieties are pendant on the polymer. This means that they are attached to the polymer at points other than at the polymer ends. Such an arrangement is readily achieved by the method used to synthesise the photoinitiators of Formula I. If m is greater than 1, the —[CR₂—CHR-Spacer(PI)_(n)]_(m) units are preferably spaced evenly along the polymer.

The value of n×m provides the total amount of photoinitiator moieties in the grafted polymer photoinitiator. Typically, polymeric photoinitiators discussed in the scientific literature have photoinitiator moieties as end groups on the polymer, whereas the present invention specifies photoinitiator moieties as pendant groups. Having photoinitiator moieties as pendant groups allows a great variation in the polymeric photoinitiator, and a higher loading of photoinitiator moieties on the polymer. This difference should also manifest itself in differences in efficiency as the polymeric photoinitiators of the present invention very quickly cure matrix compositions in comparison to other initiators. Benzophenone itself has been known to participate in the cross-linking upon curing, thus incorporating the benzophenone species into the cured cross-linked network. As such, having a significant number of pendant benzophenone groups on the grafted polymer chain provides an increase in photoinitiator efficiency.

The photoinitiator moieties of the invention may independently be cleavable (Norrish Type I) or non-cleavable (Norrish Type II). Suitably, the photoinitiator moieties of the invention are all non-cleavable (Norrish Type II). For reference, see e.g. A. Gilbert, J. Baggott: “Essentials of Molecular Photochemistry”, Blackwell, London, 1991). Upon excitation, cleavable photoinitiator moieties spontaneously break down into two radicals, at least one of which is reactive enough to abstract a hydrogen atom from most substrates. Benzoin ethers (including benzil dialkyl ketals), phenyl hydroxyalkyl ketones and phenyl aminoalkyl ketones are important examples of cleavable photoinitiator moieties. Non-cleavable photoinitiator moieties do not break down upon excitation, thus providing fewer possibilities for the leaching of small molecules from the matrix composition. The photoinitiator moieties of the invention are efficient in transforming light from the UV or visible light source to reactive radicals which can abstract hydrogen atoms and other labile atoms from polymers and hence effect covalent cross-linking. Optionally, amines, thiols and other electron donors can be either covalently linked to the polymeric photoinitiator or added separately or both. The addition of electron donors is not required but may enhance the overall efficiency of cleavable photoinitiators according to a mechanism similar to that described for the non-cleavable photoinitiators below.

Excited non-cleavable photoinitiators do not break down to radicals upon excitation, but abstract a hydrogen atom from an organic molecule or, more efficiently, abstract an electron from an electron donor (such as an amine or a thiol). The electron transfer produces a radical anion on the photoinitiator and a radical cation on the electron donor. This is followed by proton transfer from the radical cation to the radical anion to produce two uncharged radicals; of these the radical on the electron donor is sufficiently reactive to abstract a hydrogen atom from most substrates. Benzophenones and related ketones such as thioxanthones, xanthones, anthraquinones, fluorenones, dibenzosuberones, benzils, and phenyl ketocoumarins are important examples of non-cleavable photoinitiators. Most amines with a C—H bond in α-position to the nitrogen atom and many thiols will work as electron donors. The photoinitiator moieties of the invention are preferably non-cleavable.

Self-initiating photoinitiator moieties are within the scope of the present invention. Upon UV or visible light excitation, such photoinitiators predominantly cleave by a Norrish type I mechanism and cross-link further without any conventional photoinitiator present, allowing thick layers to be cured. Recently, a new class of β-keto ester based photoinitiators has been introduced by M. L Gould, S, Narayan-Sarathy, T. E. Hammond, and R. B. Fechter from Ashland Specialty Chemical, USA (2005): “Novel Self-Initiating UV-Curable Resins: Generation Three”, Proceedings from RadTech Europe 05, Barcelona, Spain, Oct. 18-20 2005, vol. 1, p. 245-251, Vincentz. After base-catalyzed Michael addition of the ester to polyfunctional acrylates, a network is formed with a number of quaternary carbon atoms, each with two neighbouring carbonyl groups.

Another self-initiating system based on maleimides has also been identified by C. K. Nguyen, W. Kuang, and C. A. Brady from Albemarle Corporation and Brady Associates LLC, both USA (2003): “Maleimide Reactive Oligomers”, Proceedings from RadTech Europe 03, Berlin, Germany, Nov. 3-5, 2003, vol. 1, p. 589-94, Vincentz. Maleimides initiate radical polymerization mainly by acting as non-cleavable photoinitiators and at the same time spontaneously polymerize by radical addition across the maleimide double bond. In addition, the strong UV absorption of the maleimide disappears in the polymer, i.e. maleimide is a photobleaching photoinitiator; this could make it possible to cure thick layers.

So, in an embodiment of the invention, the photoinitiator moieties include at least two different types of photoinitiator moieties. Preferably, the absorbance peaks of the different photoinitiators are at different wavelengths, so the total amount of light absorbed by the system increases. The different photoinitiators may be all cleavable, all non-cleavable, or a mixture of cleavable and non-cleavable. A blend of several photoinitiator moieties may exhibit synergistic properties, as is e.g. described by J. P. Fouassier: “Excited-State Reactivity in Radical Polymerization Photoinitiators”, Ch. 1, pp. 1-61, in “Radiation curing in Polymer Science and technology”, Vol. II (“Photo-initiating Systems”), ed. by J. P. Fouassier and J. F. Rabek, Elsevier, London, 1993. Briefly, efficient energy transfer or electron transfer takes place from one photoinitiator moiety to the other in the pairs [4,4′-bis(dimethyl-amino)benzophenone+benzophenone], [benzophenone+2,4,6-trimethylbenzophenone], [thioxanthone+methylthiophenyl morpholinoalkyl ketone].

Furthermore, it has recently been found that covalently linked 2-hydroxy-1-(4-(2-hydroxyethoxy)phenyl)-2-methylpropan-1-one, which is commercially available with the trade name Irgacure 2959, and benzophenone in the molecule 4-(4-benzoylphenoxyethoxy)phenyl 2-hydroxy-2-propyl ketone gives considerably higher initiation efficiency of radical polymerization than a simple mixture of the two separate compounds, see S. Kopeinig and R. Liska from Vienna University of Technology, Austria (2005): “Further Covalently Bonded Photoinitiators”, Proceedings from RadTech Europe 05, Barcelona, Spain, Oct. 18-20 2005, vol. 2, p. 375-81, Vincentz. This shows that different photoinitiator moieties may show significant synergistic effects when they are present in the same oligomer or polymer.

Each and every one of the above-discussed types of photoinitiators and photoinitiator moieties may be utilised as photoinitiator moieties in the polymeric photoinitiators of the present invention.

According to the invention, the photoinitiator moieties are independently selected from the group consisting of benzoin ethers, phenyl hydroxyalkyl ketones, phenyl aminoalkyl ketones, benzophenones, thioxanthones, xanthones, acridones, anthraquinones, fluorenones, dibenzosuberones, benzils, benzil ketals, α-dialkoxy-acetophenones, α-hydroxy-alkyl-phenones, α-amino-alkyl-phenones, acyl-phosphine oxides, phenyl ketocoumarins, silanes, maleimides and derivatives thereof. Suitably, the photoinitiator moieties are optionally-substituted benzophenones or thioxanthones. The benzophenones or thioxanthones may be “optionally substituted” with one or more moieties as defined above.

Polymeric Photoinitiators of the Invention

The polymeric photoinitiators of the invention can be synthesised by grafting the photoinitiator moieties onto a polymer backbone.

The grafting process begins from a polymer having an X—H bond, and photoinitiator moieties (PI) comprising at least one activated alkene functional group. Radical addition of an X—H bond in the polymer takes place across the C═C double bond of the activated alkene functional group(s) present on the photoinitiator moiety. X—H is a single bond present in the polymer which can form a radical, typically C—H or N—H. For this reason, in the grafted product, a hydrogen atom H is always present on the second carbon atom from the polymer, which has the formula —CHR—.

For the sake of simplicity, each photoinitiator moiety PI suitably comprises one activated alkene functional group per PI moiety.

By “activated alkene” is meant an alkene which—by means of its substituents—has an increased reactivity to radical addition reactions as compared to an alkene substituted with a hydrogen atom (H—). Substituents on the alkene may act as electron-withdrawing or electron-donating substituents, thus changing the electron-density distribution of the activated alkene. Electron-withdrawal or donation from the alkene may be via n-bonds (i.e. delocalisation of electrons), or α-bonds.

Suitably, the activated alkene functional group is selected from the group consisting of acrylate, vinyl ether, vinyl amine, acrylamide and styrene functional groups. Preferably, the activated alkene functional group is selected from the group consisting of acrylate, vinyl ether, vinyl amine and styrene functional groups. Most preferably, the activated alkene is acrylate.

The method of the invention described above allows the production of various polymeric photoinitiators.

In one aspect, the activated alkene is an acrylate functional group. By “acrylate functional group” is meant the functionality C═C—C(═O)—. In other words, the photoinitiator moiety (PI) comprising an ethylene functionality, from which the polymeric photoinitiators of formula (I) are synthesised, comprises the functional group: CR₂═CR—C(═O)—. The products of the grafting reaction of these photoinitiator moieties with polymers have the general formula II:

Polymer-[CR₂—CHR—C(═O)-Spacer(PI)_(n)]_(m)  (II)

wherein m is an integer from 1-5000; n is a real number above 0 and below 5; each R group is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups); and PI is a photoinitiator moiety.

During the grafting, the acrylate functional group is converted into the CR₂—CHR moiety in Formula (II).

Suitably, the acrylate functional group is an acrylate ester, in which case the —[CR₂—CHR—C(═O)-Spacer(PI)_(n)]_(m) units in Formula II comprise a CR₂—CHR—C(═O)—O— moiety (i.e. the spacer group comprises an oxygen atom 0 immediately adjacent the carbonyl (C═O) moiety). Such a sub-class of polymeric photoinitiators has the general formula IIa:

Polymer-[CR₂—CHR—C(═O)—O-Spacer(PI)_(n)]_(m)  (IIa)

in which n, m, R, the polymer and PI are as defined for Formula II. This formula is exemplified in Scheme 2.

Additionally, the acrylate functional group may be an acrylamide, in which case the —[CR₂—CHR—C(═O)-Spacer(PI)_(n)]_(m) units in Formula II comprise a CR₂—CHR—C(═O)—NH— moiety (i.e. the spacer group comprises a nitrogen atom N immediately adjacent the carbonyl (C═O) moiety). Such a sub-class of polymeric photoinitiators has the general formula IIa:

Polymer-[CR₂—CHR—C(═O)—NH-Spacer(PI)_(n)]_(m)  (IIb)

in which n, m, the polymer and PI are as defined for Formula II. This formula is exemplified in Scheme 3.

In addition, polymeric photoinitiators of the general formula II are preferred in which the spacer moiety comprises an aromatic ring, preferably an optionally substituted phenyl ring adjacent the carbonyl (C═O) moiety.

The photoinitiator moiety (PI) of the invention may be a mono-, di- or tri-acrylate (i.e. comprising one, two or three C═C—C(═O)— moieties, respectively). Preferably, the photoinitiator moiety (PI) monomer is a mono-acrylate.

In another aspect, the activated alkene functional group is a vinyl ether functional group. By “vinyl ether functional group” is meant the functionality CR₂═CR—O—. This provides polymeric photoinitiators of the general formula III:

Polymer-[CR₂—CHR—O-Spacer(PI)_(n)]_(m)  (III)

wherein m is an integer from 1-5000; n is a real number above 0 and below 5; each R group is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups); and PI is a photoinitiator moiety. This formula is exemplified in Scheme 4.

In a further aspect, the activated alkene functional group is a vinyl amide functional group, i.e. the functionality CR₂═CR—NR′—. This provides polymeric photoinitiators of the general formula IV:

Polymer-[CR₂—CHR—NR¹-Spacer(PI)_(n)]_(m)  (IV)

wherein m is an integer from 1-5000; n is a real number above 0 and below 5; each R group is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups); R¹ is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN and halogens; and PI is a photoinitiator moiety.

This formula is exemplified in Scheme 5.

In yet a further aspect, the activated alkene functional group is a styrene group, i.e. the functionality CR₂═CR-Ph-. A polymeric photoinitiator of the general formula IV is thus provided:

Polymer-[CR₂—CHR-Ph-Spacer(PI)_(n)]_(m)  (V)

wherein m is an integer from 1-5000; n is a real number above 0 and below 5; each R group is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups); Ph is an optionally substituted phenyl group and PI is a photoinitiator moiety.

By “optionally substituted phenyl” is meant that the phenyl group may be substituted with one or more substituents selected from C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, hydrogen, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups), amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups). Suitably, the phenyl group is unsubstituted.

This formula is exemplified in Scheme 6.

In all the structures I, II, IIa, IIb, III, IV, and V above, R may be independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl and C3-C25 cycloalkyl. Preferably, in all structures I, II, IIa, IIb, III, IV, and V, all R groups are hydrogen. In that R groups are “independently selected”, they may be the same or different in all instances, and are preferably the same.

Details of the polymer, the photoinitiator, the spacer, R, R¹, and the indices n and m are as described above.

Grafting

Briefly, the process known as “grafting” involves the radical addition of an X—H bond in the polymer across the C═C double bond of the activated alkene functional group present on the photoinitiator. X—H is a single bond present in the polymer which can form a radical, typically C—H or N—H.

The polymeric photoinitiators can be synthesized using an extruder as a reactor in a process known as reactive compounding; a technique well known in processes where vinylsilanes are grafted onto polyolefines. The photoinitiator moieties need to have an activated alkene functional group, which can react with radicals generated on the polymeric backbone through reaction with a radical initiator. Therefore, the grafting of the photoinitiator moieties (PI) onto the polymer may be promoted by a radical initiator.

This is exemplified in Scheme 7, where benzoyl peroxide is used as a radical initiator. All processes occur in the extruder.

Instead of using a radical initiator, it can be envisaged that the shear forces present during extrusion are sufficient to cause chain scission leaving free radicals which can react with the activated alkene-containing photoinitiator moieties. The skilled person will be able to tune the screw design of the extruder and to optimise process parameters for the extrusion process as required. Therefore, a method according to the invention is provided wherein the grafting of the photoinitiator moieties (PI) onto the polymer takes place in an extruder.

Several reports detail the grafting of acrylates onto polyethylenes using reactive extrusion/compounding, where for example grafting of 2-(dimethylamino) ethyl methacrylate is described in K. E. Oliphant, K. E. Russel, W. E. Baker Polymer, 36 (1995), 1597-1603. The radical initiator 1,1-di-(tert-butylperoxy)-3,3,5-trimethylcyclohexane was used in the process.

Similarly, Y. M. Krivoguz, S. S. Pesetskii, B. Jurkowski, T. Tomczyk J. Appl. Polym. Sci., 102 (2006), 1746-1754 describes how itaconic acid is grafted onto polypropylene and polyethylene also using a peroxide as radical initiator in a compounding step.

In one method according to the invention, the activated alkene functionality is an acrylate functional group.

Grafting photoinitiators onto polymeric materials using activated alkenes as a starting material provides the following benefits: (a) Photoinitiators substituted with activated alkenes (e.g. acrylates) are readily available such as 1-(4-benzoylphenyl)prop-2-en-1-one (obtained from Chemos GmbH). (b) Residual activated alkenes in the form of starting material or pure polymers obtained from reaction of the activated alkene with itself, are typically more soluble than for example a polyolefin grafted photoinitiator. This eases purification of the grafted photoinitiator.

One purpose of the present invention is to provide a method that can be used to fabricate, with sufficient ease, a polymeric photoinitiator compatible with the components of a specific matrix composition.

An example within the present invention is shown in Scheme 8, where 1-(4-benzoylphenyl)prop-2-en-1-one is grafted onto polyethylene.

The radical initiator could for example be 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (trade name L-101 manufactured by Elf-Atokem France). Other suitable initiators could be dicumyl peroxide (trade name DCP manufactured by AO Kazanorgsintez), 2,2-di-(tert-butylperoxy)-5,5,6-trimethylbiscyclo-[2,2,1]-heptane (trade name D-1 manufactured by Chemico-Technological Center of Belarus, Academy of Sciences), 2,2-di(3-methyl-1-butyne-3-peroxy)-5,5,6-trimethylbiscyclo[2,2,1]heptane (trade name D2 manufactured by Chemico-Technological Center of Belarus, Academy of Sciences) or 2,5-dimethyl-2-hydroxy-5-tert-butylperoxy-3-hexyne (trade name OP-2 manufactured by Chemico-Technological Center of Belarus, Academy of Sciences), benzoyl peroxide, lauroyl peroxide, t-butyl α-cumyl peroxide, di-t-butyl peroxide, di-t-amyl peroxide, t-butyl peroxybenzoate, t-amyl peroxybenzoate, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, α,α′-bis(t-butylperoxy)-1,3-diisopropylbenzene, α,α′-bis(t-butylperoxy)-1,4-diisopropylbenzene, 2,5-bis(t-butylperoxy)-2,5-dimethylhexane, 2,5-bis(t-butylperoxy)-2,5-dimethyl-3-hexyne.

In a previous work (Y.-J. Sun, G.-H. Hu, M. Lambla Angew. Makrom. Chem. 3982 (1995), 1-13) it has been described that addition of styrene facilitates an increase in grafting efficiency in particular, when polypropylene is decorated with acrylates, e.g. glycidyl methacrylate.

Of particular importance is the grafting onto polyethylene oxide as exemplified in Scheme 9, where 4-acryloylbenzophenone is grafted onto a PEO chain using a radical initiator.

The radical initiators suitable for the reaction depicted in Scheme 9 can for example be selected from the initiators listed above.

The present invention comprises aspects of synthesizing the polymeric photoinitiators through a reactive compounding process, where a photoinitiator precursor containing activated alkene functional groups (e.g. acrylates) is grafted onto a polymer with the aid of a radical initiator such as e.g. benzoyl peroxide.

Matrix Composition

The polymeric photoinitiators of the invention may be incorporated into a matrix composition. The polymeric photoinitiators may be the sole component of the matrix composition, or the matrix composition may comprise additional components, such as gel-forming polymers or gel-forming monomers.

Gel-forming polymers are polymers which—due to their hydrophilic nature—retain a swelling medium such as water within the polymer structure, allowing a hydrophilic gel to be formed, once the matrix composition is cured.

In particular, the gel-forming polymer may be a hydrogel-forming polymer. A hydrogel-forming polymer is selected from the group comprising polyacrylates, polyalkylethers such as polyethylene oxide, polyurethanes, polyamides, polyethylene vinyl acetates, polyvinylpyrrolidone and co-polymers and blends thereof. Preferably, the hydrogel-forming polymer is selected from the group consisting of polyalkylethers, polyurethanes, polyethylene vinyl acetate.

A gel-forming monomer is a monomer which produces a gel-forming polymer when polymerised. A hydrogel-forming monomer is one which produces hydrogel-forming polymers as set out above. Suitable hydrogel-forming monomers may be selected from the group consisting of acrylate monomers, N-vinylpyrrolidone, and epoxide monomers and, for example, monomers with two or more hydroxyl and/or amino functionalities, such as diethanol and aminoethanol.

Polymerization of the monomeric entities occurs in conjunction with cross-linking.

Other possible components in the matrix composition include anti-oxidants such as BHT (2,6-bis(1,1-dimethylethyl)-4-methylphenol), Irganox 1010 (from Ciba) and similar structures. Therapeutic additives are also possible components in the matrix composition. When such additional components are present in the matrix composition, they may be added directly at the same time as the matrix composition is formed, at any point prior to curing, or as a component of the swelling medium. The latter is most preferred.

Curing

The matrix composition of the invention is cured by exposing it to UV radiation.

Curing can either occur in the molten state, or in a solution. The latter comprises steps, where the matrix composition is dissolved in a suitable solvent and for example spray-coated on to a tube, and subsequently exposed to UV radiation. The solvent can afterwards either be evaporated or remain in the coating and function as a swelling medium to provide the desired gel.

The ultraviolet spectrum is divided into A, B and C segments where UV A extends from 400 nm to 315 nm, UV B from 315 to 280 nm, and UV C from 280 to 100 nm. By using a light source that generates light with wavelengths in the visible region (400 to 800 nm) some advantages are obtained with respect to the depth of the curing, provided that the photoinitiator can successfully cure the material at these wavelengths. In particular, scattering phenomena are less pronounced at longer wavelength, thus giving a larger penetration depth in the material. Thus photoinitiators which absorb, and can induce curing, at longer wavelength are of interest. By judicially choosing substituents on the aromatic moieties, the absorption spectrum of the polymeric photoinitiator can to some extent be red-shifted, which would then facilitate curing at comparatively greater depths.

Multi-photon absorption can also be used to cure samples using light sources emitting at wavelengths twice or even multiple times the wavelength of light needed for curing in a one-photon process. For example, a composition containing a photoinitiator with an absorption maximum at ˜250 nm could possibly be cured with a light source emitting at ˜500 nm utilizing a two-photon absorption process provided that the two-absorption cross section is sufficiently high. A multi-photon initiated cure process could also facilitate greater spatial resolution with respect to the cured area, exemplified in Nature 412 (2001), 697 where a 3D structure is formed by a two-photon curing process.

In the present invention, curing is primarily initiated by exposing the matrix composition to high energy irradiation, preferably UV light. The photoinitiated process takes place by methods described above and which are known per se, through irradiation with light or UV irradiation in the wavelength range from 250 to 500 nm. Irradiation sources which may be used are sunlight or artificial lamps or lasers. Mercury high-pressure, medium pressure or low-pressure lamps and xenon and tungsten lamps, for example, are advantageous. Similarly, excimer, solid stated and diode based lasers are advantageous. Even pulsed laser systems can be considered applicable for the present invention. Diode based light sources in general are advantageous for initiating the chemical reactions.

In the curing process the polymeric photoinitiator transforms the matrix composition in a chemical process induced by light.

Auto-Curing

The polymeric photoinitiators described here can both facilitate curing of a surrounding matrix but since the polymeric photoinitiators themselves are polymers they can also “auto-cure”, meaning that the polymeric photoinitiators can solely constitute the matrix composition that is cured with UV irradiation. This is particularly relevant when the polymer in Formula I is a polyolefin or polyalkyloxide, preferably a polyalkyloxide.

In one aspect, therefore, the invention provides a method for manufacturing a hydrophilic gel precursor, said method comprising the steps of

-   -   a. providing a matrix composition consisting of a polymeric         photoinitiator of the invention, and     -   b. curing the matrix composition obtained in step a. by exposing         it to UV radiation.

The “auto-curing” method suitably takes place with steps a. and b. directly after one another (i.e. with no intermediate steps). In one aspect of this “auto-curing” method, the method consists of steps a. and b. alone.

A one-component system—as provided by the “auto-curing” method—provides advantages, in that the polymeric photoinitiators are thermoplastic. As such, they become less viscous under higher shear rate, making them easier to process in an extrusion process. In contrast, for example, polyvinyl pyrrolidone cannot be extruded. All details and structural refinements of the polymeric photoinitiator provided herein are aimed at providing photoinitiators suitable for use in the “auto-curing” method.

In addition, the polymeric photoinitiators of the “auto-curing” method may comprise the sole component of the matrix composition; i.e. the matrix composition may consist of the polymeric photoinitiators. This provides the advantage that additives (e.g. plasticizers, viscosity modifiers) can be avoided, thereby reducing the chances of low molecular weight components from leaching from the cross-linked matrix composition.

Gel-State

A gel is characterized as a swellable material, however, insoluble in the swelling medium. By hydrogel is meant a material comprised mainly of a water soluble or water swellable material. The gel material is characterized in terms of its rheological properties and in its dry state. In particular, the storage and the loss modulus are used to characterize the mechanical properties of the materials (T. G. Mezger: “The Rheology Handbook”, Vincentz Network, Hannover, 2006). As described above, curing of a matrix composition is followed by monitoring the change of G′(ω) and G″(ω) as a function of UV exposure time. In the examples used to describe the present invention, a frequency of 1 Hz is used to probe the rheological properties and the samples were heated to 120° C. during testing.

The invention also relates to a hydrophilic gel, obtainable via the methods described herein.

The invention also relates to the use of a photoinitiator, of the general formula I, II, IIa, IIb, III, IV or V, wherein m, n, PI, R, R¹, the Spacer and the polymer are as defined herein, in the manufacture of a hydrophilic gel, in particular a hydrogel.

Example 1

A series of polyethylene grafted polymers were made by charging a Brabender mixer with 60 g of Flexirene MS 40A at 150° C. To this mixture was added various amounts of 4-acryloylbenzophenone (1-10 g) and 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane in various amounts (0.5-5 g). This mixture was blended for 20 minutes at 150° C. and subsequently collected from the Brabender mixer. The isolated material was analyzed using FT-IR to ensure complete conversion of the added acrylate.

Example 2

Further grafting experiments were done on PEO samples using Lauroyl peroxide as the radical initiator. The formulations are summarized in Table 1.

Formulation PEO 4-benzoylphenyl acrylate Lauroyl peroxide 1 1500 g 30 g  7.5 g 2 1500 g 30 g 15.0 g 3 1500 g 30 g 47.4 g

A premix of each of these formulations was made at room temperature using a fluid mixer (FM/L4 from Thyssen Henschel) with a volume of 4 liters and operated at 300 rpm. The premixed compounds were further processed in a twin-screw extruder (ZSE 27 MAXX, Leistritz, Germany) with a length of 44D. The temperature was 80° C. at the lowest end and 130° C. in the segments with the highest temperature. The powder mixtures were added into the hopper using a gravimetric dosing system. To attain a sufficient residence time for the reaction, a throughput of 6 kg/h was optimized at a screw speed of 100 rpm. With these parameters the trials could be carried out without processing problems.

The materials were pelletized and heat-pressed into ˜1 mm sheets. An oblate of the particular sample under investigation was placed between the two plates in a rheometer (parallel plate configuration, bottom plate is a quartz glass plate). The distance between the plates was set to 0.3 mm and the temperature to 120° C. The measurements were run with fixed strain of 1% and a constant frequency of 1 Hz. When the loss and storage modules had stabilized, a UV-lamp was turned on, thus irradiating the sample through the bottom plate on the rheometer via a fibre from the lamp. The loss and storage modules were then followed as a function of time, while the UV-lamp irradiated the sample. An illustrative result of such a measurement is shown in FIGS. 2 and 3, where the change in rheology of samples of 1 and 3 is depicted as a function of exposure time.

Initially, the loss modulus is larger than the storage modulus showing liquid behaviour of the melt for both samples. When the UV source is turned on, the moduli increase, suggesting that cross-linking reactions are initiated which forms a covalently-bonded network of polyethylene oxide chains. Eventually, the storage modulus becomes larger than the loss modulus showing that a solid or gel has formed as a result of the curing reaction.

In a visual inspection, the cross-linked polyethylene oxide oblate was placed in water which resulted in a swelled gel.

Although the invention has been described with reference to a number of embodiments and preferred features, the fullest scope should not be considered as limited to precisely these embodiments. The scope of the invention is determined by the appended claims. Compatible features from various embodiments and aspects of the invention may be combined at will by the person skilled in the art, while remaining within the framework of the invention. 

1. A method for producing a polymeric photoinitiator, of the general formula I: Polymer-[CR₂—CHR-Spacer(PI)_(n)]_(m)  (I) wherein m is an integer from 1-5000; n is a real number above 0 and below 5; and PI is a photoinitiator moiety; and wherein each R group is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups), said method comprising grafting one or more photoinitiator moieties (PI), each of which comprises at least one activated alkene functional group, onto the polymer.
 2. The method according to claim 1, wherein the activated alkene functional group is selected from the group consisting of acrylate, vinyl ether, vinyl amine, acrylamide and styrene functional groups.
 3. The method according to claim 2, wherein the activated alkene functional group is selected from the group consisting of acrylate, vinyl ether, vinyl amine and styrene functional groups, and is preferably acrylate.
 4. The method according to claim 1, wherein the polymer is selected from the group consisting of polyolefines, polyacrylates, polyesters, polyurethanes, polyamides, polyalkyloxides or various copolymers comprising blocks or repeatable units of these polymers.
 5. The method according to claim 4, wherein the polymer is selected from the group consisting of polyolefines, polyacrylates, polyesters, polyurethanes, polyamides, and polyalkyloxides.
 6. The method according to claim 5, wherein the polymer is a polyolefin or polyalkyloxide.
 7. The method according to claim 1, wherein the polymer has a molecular weight in the range of 50-500,000 Da.
 8. The method according to claim 1, wherein the spacer is selected from the group consisting of a single bond, C1-C25 linear alkylene, C3-C25 branched alkylene, C3-C25 cycloalkylene, arylene, heteroarylene, amines, amides, alcohols, ethers, esters, thioethers, sulfones and derivatives thereof, sulfonic esters and derivatives thereof, sulfoxides and derivatives thereof and carbonates, ketones and esters.
 9. The method according to claim 8, wherein the spacer is selected from the group consisting of C1-C25 linear alkylene, C3-C25 branched alkylene, C3-C25 cycloalkylene, and esters.
 10. The method according to claim 1, wherein the spacer is a polymeric entity.
 11. The method according to claim 10, wherein the spacer is selected from the group consisting of polyolefines, polyacrylates, polyurethanes, polyesters, polyamides, polydialkylsiloxanes, polyalkyloxides or various copolymers comprising blocks or repeatable units of the mentioned polymers.
 12. The method according to claim 10, wherein the spacer has a molecular weight in the range of 50-500,000 Da.
 13. The method according to claim 1, wherein the photoinitiator moiety is independently selected from the group consisting of benzoin ethers, phenyl hydroxyalkyl ketones, phenyl aminoalkyl ketones, benzophenones, thioxanthones, xanthones, acridones, anthraquinones, fluorenones, dibenzosuberones, benzils, benzil ketals, α-dialkoxy-acetophenones, α-hydroxy-alkyl-phenones, α-amino-alkyl-phenones, acyl-phosphine oxides, phenyl ketocoumarins, silanes, maleimides and derivatives thereof.
 14. The method according to claim 13, wherein the photoinitiator moiety is optionally substituted benzophenone or thioxanthone.
 15. The method according to claim 1, wherein m is an integer from 1-3000, preferably from 1-1000.
 16. The method according to claim 1, wherein the grafting of the photoinitiator moieties (PI) onto the polymer is promoted by a radical initiator.
 17. The method according to claim 1, wherein the grafting of the photoinitiator moieties (PI) onto the polymer takes place in an extruder.
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 39. A method of curing a matrix composition, said method comprising the steps of: a. providing a matrix composition comprising the polymeric photoinitiator according to any of the general formulas (II), (IIa), (IIb), (III), (IV), or (V), Polymer-[CR₂—CHR—C(═O)-Spacer(PI)_(n)]_(m)  (II) Polymer-[CR₂—CHR—C(═O)—O-Spacer(PI)_(n)]_(m)  (IIa) Polymer-[CR₂—CHR—C(═O)—NH-Spacer(PI)_(n)]_(m)  (IIb) Polymer-[CR₂—CHR—O-Spacer(PI)_(n)]_(m)  (III) Polymer-[CR₂—CHR—NR¹-Spacer(PI)_(n)]_(m)  (IV) Polymer-[CR₂—CHR-Ph-Spacer(PI)_(n)]_(m)  (V) wherein m is an integer from 1-5000; n is a real number above 0 and below 5; each R group is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups); and PI is a photoinitiator moiety; R¹ is a substituent independently selected from hydrogen, C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN and halogens; and Ph is an optionally substituted phenyl group; b. curing the matrix composition obtained in step a. by exposing it to UV radiation.
 40. The method according to claim 39, wherein the matrix composition consists of the polymeric photoinitiator.
 41. A cured matrix composition obtainable via the method of claim
 39. 42. The method according to claim 39, wherein the Ph group is substituted with one or more substituents independently selected from C1-C25 linear alkyl, C3-C25 branched alkyl, C3-C25 cycloalkyl, aryl, heteroaryl, —OH, —CN, halogens, amines (e.g. —NR′R″, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups) and amides (e.g. —CONR′R″ or R′CONR″—, where R′ and R″ are alkyl groups, suitably C1-C25 alkyl groups),
 43. The method according to claim 39, wherein the polymer is selected from the group consisting of polyolefines, polyacrylates, polyesters, polyurethanes, polyamides, polyalkyloxides or various copolymers comprising blocks or repeatable units of these polymers.
 44. The method according to claim 43, wherein the polymer is selected from the group consisting of polyolefines, polyacrylates, polyesters, polyurethanes, polyamides and polyalkyloxides.
 45. The method according to claim 39, wherein the polymer is a polyolefin, or polyalkyloxide.
 46. The method according to claim 39, wherein the polymer has a molecular weight in the range of 50-500,000 Da.
 47. The method according to claim 39, wherein the spacer is selected from the group consisting of C1-C25 linear alkylene, C3-C25 branched alkylene, C3-C25 cycloalkylene, arylene, heteroarylene, amines, amides, alcohols, ethers, esters, thioethers, sulfones and derivatives thereof, sulfonic esters and derivatives thereof, sulfoxides and derivatives thereof and carbonates, ketones and esters.
 48. The method according to claim 47, wherein the spacer is selected from the group consisting of C1-C25 linear alkylene, C3-C25 branched alkylene, C3-C25 cycloalkylene, and esters.
 49. The method according to claim 39, wherein the spacer is a polymeric entity.
 50. The method according to claim 49, wherein the spacer is selected from the group consisting of polyolefines, polyacrylates, polyurethanes, polyesters, polyamides, polydialkylsiloxanes, polyalkyloxides or various copolymers comprising blocks or repeatable units of the mentioned polymers.
 51. The method according to claim 49, wherein the spacer has a molecular weight in the range of 50-500,000 Da.
 52. The method according to claim 39, wherein the photoinitiator moiety is independently selected from the group consisting of benzoin ethers, phenyl hydroxyalkyl ketones, phenyl aminoalkyl ketones, benzophenones, thioxanthones, xanthones, acridones, anthraquinones, fluorenones, dibenzosuberones, benzils, benzil ketals, α-dialkoxy-acetophenones, α-hydroxy-alkyl-phenones, α-amino-alkyl-phenones, acyl-phosphine oxides, phenyl ketocoumarins, silanes, maleimides and derivatives thereof.
 53. The method according to claim 52, wherein the photoinitiator moiety is benzophenone or thioxanthone.
 54. The method according to claim 39, wherein m is an integer from 1-3000, preferably from 1-1000.
 55. The method according to claim 39, wherein the photoinitiator units are pendant on the polymer. 